This invention relates to the inspection of flat electronic circuit patterned media at various stages of production using electrical, electro-optical and optical techniques. More specifically, this invention relates to the automated electrical, electro-optical and optical inspection of flat electronic substrates, such as thin film transistor (TFT) arrays (the main component of the liquid crystal flat panel displays (LCD)). In particular, the focus is on the inspection of high density thin film transistor (TFT) liquid crystal display (LCD) panels, deposited on large sheets of glass.
During the manufacturing of TFT LCD panels, large clear sheets of thin glass are used as a substrate for the deposition of various layers of materials to form electronic circuits that are intended to function as a plurality of separable, identical display panels. This deposition is usually done in stages where, in each stage, a particular material (such as metal, Indium Tin Oxide (ITO), Silicon, Amorphous Silicon etc.) is deposited over a previous layer (or upon the bare glass substrate) in adherence to a predetermined pattern. Each stage includes various steps such as deposition, masking, etching and stripping.
During each of these stages and at various steps within a stage, many production defects may occur that have electronic and/or visual implications on the final LCD product performance. Such defects include for example circuit shorts, opens, foreign particles, misdeposition, feature size problems, over- and under-etching. The most common defects, shown in FIG. 1, include metal protrusion 110 into ITO 112, ITO protrusion 114 into metal 116, a so-called mouse bite 118, an open circuit 120, a short 122 in a transistor 124, and a foreign particle 126.
In the primary intended application domain of TFT LCD panel inspection, the article defects that are the subject of detection are small (down to individual micrometers) requiring demanding defect detection limits.
Mere detection of defects is insufficient. Detected defects must also be classified as process defects, i.e., minor imperfections which do not undermine the performance of the finished product but which are an early indication of the array manufacturing process drifting out of optimum conditions, reparable defects which can be repaired to improve the array production yield, and finally killer defects which disqualify the TFT array from further use.
A broad range of techniques have been used to detect and classify the aforementioned defects. The techniques belong to the three broad categories, namely electrical, electro-optical and purely optical.
Electrical and electro-optical techniques require the subject of inspection to have some measurable electrical or electromagnetic properties. This is the case for the primary intended application domain of inspecting TFT LCD panels. The subject can be excited by electrical means, and the resulting electrical or electromagnetic behavior be measured and recorded. The behavior is then compared with the known normal behavior to determine the presence or absence of abnormalities in the subject being inspected.
For these techniques, the physical size or visibility of the defect causing the electrical and/or electromagnetic anomaly is often not a limiting factor for detection. The defect will be detected as long as it has a significant, measurable impact on the electrical and/or electromagnetic behavior of the circuit on the subject being inspected. The inherent limitation of this type of inspection however is that only defects that affect the electrical and/or electromagnetic behavior of the circuit can be detected. Other anomalies, independent of size, are missed by the inspection system. In addition, some of these defects may affect an area which is much larger than the physical defect size. Hence, in some common cases, an electrical or electro-optical inspection technique would detect a defect signature which does not necessarily correspond to the physical defect that is the source of the anomaly on the subject being inspected.
Purely optical inspection techniques, referred to as automated optical inspection (AOI), can both detect and locate defects independent of their electrical properties, provided they are visible under the chosen optical setup. However, there is often a significant limit on the size of the defects that can be detected in a reasonable amount of time. Better detection limits (smaller defects) and shorter inspection times are both very important goals. However for AOI systems, these are always conflicting goals and must be compromised. What is achievable is limited by the available technology of imaging and processing hardware. The inspection time also scales up with the size of the article being inspected. Therefore, for AOI applications, the resulting high sensitivity systems are often slow and faster systems have coarser imaging resolutions and hence lower detection sensitivities.
Electrical and electro-optical inspection methods are particularly attractive because of their ability to detect electrical defects even when they are very small or when they are buried under other layers of deposition and hence are optically invisible. This is especially true for the final inspection of the TFT LCD panels after all layers of material have been deposited, the LCD panel is fully functional and can be electrically excited. Therefore, providing an improvement for overcoming the defect localization and classification limitations of the electrical and electro-optical inspection methods is an important problem.
There is a variety of methods and apparatus in the prior art for detecting abnormalities in flat patterned media and in particular in glass plates deposited with TFT LCD flat panels. A first category of such methods is the electrical testing of the circuitry which is formed by the material deposition on the flat media. These technique can be used when the pattern in question forms a complete or partial electrical circuit that can be excited and whose electrical variables can be measured.
In an article “Functional Testing of TFT/LCD Arrays”, IBM Journal of Research & Development, Vol. 36, No. 1, January 1992; Jenkins et al. argue that the defects in charge holding capabilities of individual TFT LCD pixels, as well as the presence of a number of other defects, can be detected by selectively charging the pixels and discharging them through a sensing circuit. In this way, the measurement is of how much of the originally stored charge can be recovered.
A sensing aspect of the method and its connections to the LCD panel pixels are described in detail in U.S. Pat. No. 5,179,345 to Jenkins et al. A TFT LCD panel defect detection apparatus is described in U.S. Pat. No. 5,546,013 to Ichioka et al.
The aforementioned testing method requires galvanic connections to all gate and data lines of a TFT LCD panel structure to be able to address and electrically test individual pixels. This limitation makes for contact hardware that is expensive and difficult to maintain. An improvement over the method to reduce the number of required contacts is proposed in U.S., Pat. No. 6,437,596 B1 to Jenkins et al.
Another type of electrical testing of such circuits on flat media is by means of electron beams irradiated to the surface being inspected. A number of solutions using electron beams make use of the basic principles of Scanning Electron Microscopy (SEM). In this method, by means of irradiating the circuit features with a low energy electron beam and recording the energy level of the secondary electrons scattered from the said circuit features, a voltage distribution on the surface being inspected can be generated. This generated voltage map can be used to detect the presence and type of electrical defects on the surface.
U.S. Pat. No. 4,843,312 to Hartman et. al., describes the general approach of using an energy beam (or particle beam) to test for the defects in an TFT LCD circuit structure.
U.S. Pat. No. 5,414,374 to Brunner et al. also describes a particle beam tester for LCD panels. In this case, the plane electrodes of the LCD pixels are targeted by the beam to create a potential on the electrode. In a following measurement cycle, this potential is measured by means of the scattered secondary electrons of either the same or a second beam and compared to the nominal values. The switching element is also activated during the measurement period to provide a time varying potential, which can also be measured and compared to the nominal behavior.
Another rather different application of the electron beam testing is through charging or discharging the circuit features by means of the electron beam sequentially directed to localized sections of the said circuit features. For example, the individual pixels in a TFT LCD panel pixel array are excited with a low energy electron beam and the induced currents measured to determine the presence of electrical defects in the structure. In an article named “Electron-Beam Technology for Open/Short Testing of Multi-Chip Substrates,” IBM Journal of Research & Development, Vol. 34, No. 2/3, March/May 1990, Golladay et al. describe this technique. A more recent U.S. Pat. No. 5,612,626 to Golladay et al., describes an electronic substrate defect detection system based on electron beam technology.
The second category of defect detection methods is that of electro-optical techniques. In this category of methods, the electrical and/or electromagnetic behavior of the circuit is converted into an optically observable (typically through visible light or e-beam reflection formed through a suitable modulator. The output of the modulator is then imaged through optical means to form images representative of the electrical behavior of the inspected circuits. In particular, the images of the electric field and hence the voltage distribution of the inspected circuits can be generated in the form of voltage maps These images can then be used to detect and identify the electrical defects on the surface.
U.S. Pat. Nos. 4,983,911, 5,097,201 and 5,124,635 to Henley describe the principles of an electro-optical light modulator and associated imaging means where the voltage and the resulting electric field from a surface under test are used to modulate the optical properties of a suitable liquid crystal variant structure. An optical image is thus created and recorded by means of an image capture device. An improved version of this electro-optical modulator with its manufacturing method is disclosed in a later U.S. Pat. No. 5,615,039 to Henley.
U.S., Pat. No. 5,570,011 to Henley describes a complete method of using this electro-optical sensing element in a testing arrangement for testing electronic devices, which can be excited to be in known electrical states. This method has been successfully used in the electro-optical testing of the electrical functionality of the TFT LCD panels at the final stages of their production before being assembled and filled with liquid crystal.
The other major category of defect detection methods, Automated Optical Inspection (AOI) is based on optical imaging at the required magnification and resolution and using hardware/software image processing techniques to detect defects out of the expected usual variations of the article being inspected. The anomalies detected are not limited to electrically significant defects but rather they are limited to optically discernible objects determined by the given optical configuration, the imaging magnification and the resolution of the system. The resolution also determines the complexity and the operational speed of such a system.
The basic principle of performing such an automatic inspection is based on imaging the article being inspected with a chosen magnification and resolution and then digitizing the image information using an image capture device such as a CCD or CMOS sensor. The captured images may be used to build references of the normal variations of the surface of the article being inspected and perform defect detection based on comparisons with a reference. The comparison procedure may be done in the spatial domain where spatial pattern comparison techniques such as image subtraction are used to detect the deviation of a test image from a reference. Alternatively, the comparison can be done in the feature domain with suitably chosen representative features. In this latter case, both the test article and reference are represented by sets of features derived from the images. The comparison is also performed in this chosen feature space.
There exist many approaches for optical inspection. For example, In U.S. Pat. Nos. 4,247,203 and 4,347,001 to Levy et al. describe an automatic optical photomask inspection apparatus using a spatial pattern comparison technique to detect defects in photomasks of repeating circuit dies.
U.S. Pat. No. 4,805,123, to Specht et al., discloses an improved method of inspecting surfaces with such repetitive patterns by means of spatial pattern comparison at sub-pixel resolution. A detection sensitivity close to the imaging resolution is achievable through careful sub-pixel alignment of the reference and test images. Such detection sensitivity is normally not possible because of the aliasing noise (also called pixelation noise) in the image sampled at a particular resolution. The method has been used in silicon wafer inspection as well as TFT LCD inspection.
There have also been other solutions proposed for photomask and integrated circuit die pattern inspection. These have been described for example in U.S. Pat. No. 4,926,489 to Danielson et al., U.S. Pat. No. 5,864,394 to Jordan et al., and U.S. Pat. No. 6,282,309 to Emery.
Application of the automated optical inspection technology to the inspection of TFT LCD panels has consisted of the scaling up of the techniques well established for the inspection of integrated circuit dies. However, other techniques addressing specific problems of the application domain, such as improving material contrast have also been disclosed. For example, U.S. Pat. No. 5,333,052 to Finarov describes a phase contrast imaging technique which is especially useful for improving the contrast, and hence the detectability of transparent materials (such as ITO) in TFT LCD panels.
AOI systems may be capable of detecting and imaging defects of electrical nature and of non-electrical nature and hence can be used for process control purposes by detecting defects which do not immediately lead to functional failures. However, their performance and inspection time is a function of the instrument operating resolution.
Because many defects causing severe electrical failures in circuitry are very small as compared to the entire surface of the article being inspected, the requirement to detect these severe defects leads to instruments with high optical operating resolution. This, in turn, results either in a very expensive instrument, a very slow instrument or both. Even with the most expensive hardware, there is a limit on the achievable inspection speed determined by the available hardware technology. These limitations are especially significant for the preferred application area of TFT LCD panel inspection where the sizes of flat material plates deposited by TFT LCD panels are getting much larger than the circuit feature sizes involved.
Therefore, there is demand for the aforementioned electrical and electro-optical inspection technologies which can detect electrically significant defects independent of their physical sizes. However, the electrical and electro-optical systems face difficulty: For certain types of defects, the electrical signature of the defect may cover a much larger zone than the physical cause of the defect. For example, a short from a data line to the common line in an TFT LCD panel circuit will ground the entire data line and hence will result in a defect signature which covers the entire zone occupied by the data line. This inspection does not give localization information for the defect, and it is a major drawback in such systems. Defect localization information is useful in order to monitor re-occurrences of defects for process control purposes in order for an operator to manually review the nature of the defect through an optical microscope, or for an automated classification sub-system to image and process the defect. If a repair instrument is involved in the process, the localization information is further used to find and repair the defect.
An earlier U.S. patent application Ser. No. 10/223,288 to Clark et al., entitled “Integrated Visual Imaging and Electronic Sensing Inspection Systems” discloses the generic concept of augmenting a non-AOI inspection instrument with an integrated visual imaging (AOI) channel to concurrently perform the scan of the entire surface of the subject being inspected by both independent channels with the aim of combining the detection and classification results.
The typical prior art process flow for defect detection and repair is outlined in FIG. 2. The process is valid not only for Non-AOI inspection systems but also for AOI systems. In operation the article being inspected arrives from the previous process step ( Step 210) and is directed to the defect detection system 212 to be scanned for production anomalies in the article. After the defect detection system operates on the article, an output is generated that is a list of defects which are identified on the article being inspected.
Depending on the capabilities of the defect detection system, the defects may be precisely localized defect points or loosely localized defect zones which may cover a large area. For example, certain electrical testing methods are not able to determine the precise location of a line short, since the resulting electrical anomaly affects the entire line and not only the area where the physical defect occurred.
In the event no defect could be identified (Step 214) by the defect detection step (in system 212), the article is passed to the next process step (Step 224). If one or more defects are identified, then a decision is made about whether each of these defects is repairable (Step 216) by an appropriate repair system 218. Depending on the capabilities of the inspection and repair systems 212, 218, this decision can be made either in the inspection system 212 or in the repair system 218. For example, some inspection systems have the capability to perform automatic review and classification based on user specifications to classify the defects based on whether they are repairable or not.
Repairable defects are processed by the repair system and if successful (Step 228), the inspected panels are either passed to the next process step (Step 224) or are subjected to an alternative processing (Step 220) and are diverted to an alternative process flow (Step 226). The exact handling of non-repairable defective samples is usually manufacturing plant dependent. Depending on the plant strategy, alternative processing and subsequent process flow may simply lead to scrapping of the entire substrate plate, scrapping only the defective panel on the substrate or stripping and recycling the entire substrate plate to the beginning of the process. While the articles being inspected are propagated through the process, the information about the identified defects is stored and communicated through a database 228 accessible to both the inspection and repair systems.
What is needed is a panel inspection system that specifically addresses the limitation of most of the non-AOI systems such as those based on electrical and electro-optical techniques, to accurately locate certain type of defects before these defects are reviewed by the user or by an automated classifier.
Only one such technology has been introduced in the industry. In Korea, Charm, Inc. has recently developed a station to automatically locate defects in a panel in order to facilitate repair. The apparatus is an improvement in a process operating in a repair instrument. The apparatus uses a separate TDI line-scan camera as separate associated optics operating independently of the main review and repair optics of the repair station. The apparatus employs a coarse object plane pixel resolution such that sub-pixel interpolation is required to secure a desired fineness of resolution and detection sensitivity. The imaging is one-dimensional in nature, with two notable results. The image cannot be captured instantaneously and a two different scans at orthogonal angles must be performed to detect defects along two directions. Therefore, the line scan camera itself must be physically rotated between the two scanning directions.